Synthesis of the Lewis Adduct of Cu^II with Imidazole; Stoichiometry of Reaction

• To improve your skill in observation
• To synthesize a chemical substance and purify it by recrystallization
• To determine the stoichiometry of a chemical reaction

Chemical synthesis--the art and practice of the design and creation of molecules--is the essence of chemistry. In no other field of science does the practitioner have the ability to create the objects of his/her interest. It is this single fact that sets chemistry apart from physics, geology, and biology, in which the scientist is restricted to examining what s/he finds, rather than what s/he creates. Chemistry has an extra dimension that combines vision and art in the process of creating new substances with potentially new and exciting uses.

This experiment provides you with an opportunity to experience chemical synthesis first hand, to determine the composition of the molecule that you create, and to spectroscopically characterize the molecule (i.e., to discover what types and energies of light the molecule interacts with). The reaction that you will carry out is shown (sans stoichiometry) in equation (1).

The reaction will be carried out in ethanol, a commonly-used laboratory solvent. C₃H₄N₂ is the formula for imidazole (click to see the structure). This reaction is yet another example of a class of reaction that permeates chemistry: the donation of an electron pair by a Lewis base to a Lewis acid to form an adduct in which acid and base are joined by a covalent bond. As we have seen and will continue to see, the donation/acceptance of electron pairs is involved in a huge number of chemical reactions ranging from simple laboratory processes to the complex reactions in biological systems.

The adduct of Cu²⁺ with imidazole is called a transition metal complex to indicate that it is a complex molecule involving an electron pair donor and a transition metal ion (Cu²⁺). Imidazole is an important biological molecule. It occurs in the side chain of the amino acid, histidine, and very frequently donates an electron pair to a transition metal such as Cu²⁺, Fe²⁺ in enzymes and proteins.

Light Absorption and Beer's Law. Many molecules absorb light in the visible and/or ultraviolet regions of the electromagnetic spectrum, between 1100 and 200 nm. The wavelengths of light absorbed and the extent of absorption by a substance are measured using an electronic absorption spectrometer. To carry out such a measurement, we

• Prepare a solution of the substance in an appropriate solvent;
• Place the solution in a cuvette, or cell;
• Place the cell in the spectrometer;
• "Run" the spectrum.

A spectrum is a plot produced by the spectrometer of the amount of light absorbed (the absorbance) as a function of the wavelength of the light. It typically appears as shown in Figure 1. The amount of light absorbed by the substance--the Absorbance--at a particular wavelength is directly proportional to

• the length of solution through which the light beam passes, l;
• the concentration of substance (number of molecules or moles per unit volume) in the solution, M:

where A = absorbance (amount of light absorbed); and l = the length of solution through which the light passes, in cm. Usually we use a cell of 1 cm path length, so that l = 1 cm; M = the molarity (number of moles of substance per L) of the solution; and e = the proportionality constant relating A to l and M. This is called the molar absorptivity of the substance at the particular wavelength. According to equation (2), the amount of light absorbed is directly proportional to the amount of substance per unit volume of solution. This is a statement of Beer's Law.

The Molar Absorptivity. This is a quantity characteristic of substance and wavelength. Values of e for a given substance are usually quoted for the wavelengths of maximum absorbance, l_max--that is, the tops of the absorption bands. The value of e gives a measure of the effectiveness of a molecule of substance at capturing a photon of light. Values can range from as low as 0.01 to as high as 10⁵ L/mole-cm. Note that if we substitute 1000 cm³ for 1 L, the units of e reduce to cm²/mole. This shows that e is a measure of the effective cross-sectional-area of the substance with respect to capturing light. Since the e values of a substance are characteristic of the substance, we may use the amount of light absorbed to measure the concentration of the substance in a solution.

The product of reaction (1), Cu(imidazole)_x(NO₃)₂, is colored--that is, it absorbs light in the visible region (300-700 nm) of the spectrum. As a class, we will examine the amount of this product formed as a function of the relative amounts of Cu(NO₃)₂ and imidazole present in a solution. Our measure of the amount of product formed will be the amount of light that it absorbs--the absorbance--at a wavelength of 600 nm.

1. In what molar ratio are the two reactants, copper nitrate 2.5hydrate and imidazole, initially mixed?
2. Choose an appropriate wavelength and use a spreadsheet to make a plot of the absorbance at this wavelength versus the mole ratio of imidazole to copper. What is the stoichiometry of the adduct?
3. Propose a 3-dimensional structure for the adduct, keeping in mind that the x imidazole molecules tend to arrange themselves symmetrically around the copper(II) ion.
4. In narrative form, discuss the infrared spectrum of the adduct. Attempt to assign observed bands to particular structural features of the adduct.

• Labkit
• Kimwipes
• aspirator with trap
• ramrod
• Bunsen burner
• Ring stand/ring/wire gauze
• filter paper
  
  
• Cu(NO₃)₂.2.5H₂O
• imidazole, C₃H₄N₂

Safety goggles must be worn at all times in the laboratory. Be very careful around open burner flames.

Record your observations in your notebook.

Synthesis. Obtain the required equipment. If necessary, wash the glassware using brushes and Alconox detergent, then thoroughly rinse with water to remove all traces of Alconox.

Weigh 0.115 g of Cu(NO₃)₂.2.5H₂O and place in a 25-mL Erlenmeyer flask. Add 5 mL of ethanol and swirl to dissolve the copper salt completely. Record any observations in detail. Weigh 3 mmole of imidazole and add it to the Erlenmeyer flask containing the solution of copper nitrate. Swirl the flask to dissolve the imidazole. Record any observations in detail. Place the flask in an ice bath for 10-15 minutes to allow the product to crystallize. Using a flat-ended spatula or a stirring rod, dislodge the crystals from the sides and bottom of the flask, then let stand for another 5 minutes. Suction filter the mixture to isolate the solid, and suction it dry.

Purification. (The instructor will tell you whether or not to carry out the purification.) Transfer the product to a small Erlenmeyer flask or beaker and add 1-2 mL of distilled water. Gently heat the mixture to dissolve all or most of the solid, then filter the hot solution through a Kimwipe-plugged Pasteur pipet into another small Erlenmeyer. Place the filtrate in an ice bath. You may have to use the "scratch" technique to initiate crystallization. When crystallization is complete, collect the product by suction filtration. Wash it twice with 2-mL portions of acetone, suction it completely dry, and determine the mass of product obtained.

Transfer your product to a small vial and turn it in to the instructor before leaving lab.

Stoichiometry. Stoichiometry will be determined using Job's method. Label 8 small test tubes with numbers 1-8 using a Sharpie marker, and place in a test tube rack. Weigh precisely 0.5 mmole of Cu(NO₃)₂.2.5H₂O and dissolve it in exactly 10 mL of distilled water in a clean and dry 25-mL Erlenmeyer flask. Weigh precisely 0.5 mmole of imidazole and dissolve in exactly 10 mL of distilled water in a second 25-mL Erlenmeyer flask. Using a syringe pipet pump and a 2-mL graduated pipet, transfer respectively 0.1, 0.2, 0.30, 0.40, 0.60, 0.80, 1.0, and 1.2 mL of copper solution to tubes 1-8. Then transfer respectively 1.4, 1.3, 1.2, 1.1, 0.9, 0.7, 0.5, and 0.3 mL of imidazole solution to tubes 1-8. Thoroughly mix the contents of each tube. For each tube in turn, measure the absorbance of the solution at 616 nm. Determine the stoichiometry of the Lewis adduct from your data. When finished, clean all test tubes, flasks, and cuvettes. Pour copper solutions into the heavy metal waste jar.

Characterization. If desired, you may characterize your product using infrared and uv-visible spectroscopies. See your instructor for assistance.

Clean-up. When you have finished all of your work:

• Clean Labkit glassware by recommended procedures, shake off excess water, and return to the Labkit.
• Clean Pasteur pipets, graduated pipets, and vials by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Disposal Methods

Pour all solutions containing copper in the heavy metal waste jar.

Preparation Questions